An unmanned Antares rocket has exploded seconds after lift-off from a commercial launch pad in Virginia, marking the first accident since NASA turned to private operators to deliver cargo to the International Space Station.

The rocket, which was 14 storeys tall and built and launched by Orbital Sciences Corp, bolted off its seaside launch pad at the Wallops Flight Facility carrying a Cygnus cargo ship for the space station. It exploded seconds later.

"The Antares rocket suffered an accident shortly after lift-off," NASA mission control in Houston said, describing the event as a "catastrophic anomaly".

The rocket lifted off before exploding into a massive orange fireball, vivid against the night sky, and smashing into the ground.

NASA mission commentator Dan Huot said it was not known what caused the accident.

Orbital Sciences said in a statement: "We've confirmed that all personnel have been accounted for. We have no injuries in the operation today."

NASA launch control said damage appeared to be limited to the launch facility and rocket. The Antares rocket had been launched successfully on four previous missions.

Mike Pinkston, Orbital Sciences' Antares program manager, said the spacecraft was carrying "some classified cryptographic equipment, so we do need to maintain the area around the debris in a secure manner".

Rocket explosion shook buildings kilometres away

Ronda Miller, manager of the nearby Ocean Deli, said she felt the force of the blast from the eatery, about eight kilometres from the launch pad.

"We were standing outside waiting for it to launch and we saw bright red, and then we saw a big black cloud, and it shook the whole building where we work at," Ms Miller said.

"And then I came back in to work and seen fire trucks going every which way."

Launch had been delayed one day after a boat sailed into a restricted safety zone beneath the rocket's intended flight path.

Virginia-based Orbital Sciences is one of two companies hired by NASA to fly cargo to the station after the space shuttles were retired.

After the US space shuttle program ended in 2011, leaving no government program to send humans to the space station, private companies raced to restore US access.

This was the first night-time launch of an Antares rocket, Orbital said.

Engineers said the countdown had gone smoothly and there were no issues apparent with the machinery.

This planned flight was to be the third of eight under the company's $1.9 billion contract with NASA.

The second US supply line to the station is run by privately owned Space Exploration Technologies, or SpaceX, which is preparing for its fourth flight under a separate, $1.6 billion NASA contract.

Outfitted with a new, more powerful upper-stage engine, the Antares rocket launched carried a Cygnus spacecraft packed with 2,293 kilograms of supplies for the six astronauts living at the International Space Station, along with science experiments and equipment, a 15 per cent increase over previous missions.

No cargo that was absolutely critical was lost and the research outpost had enough consumables to last until another delivery could be made, NASA flight facility director Bill Robel said.

Cygnus was to loiter in orbit until November 2, then fly itself to the station so astronauts could use a robotic crane to snare the capsule and attach it to a berthing port.

The station, a $100 billion research laboratory owned and operated by 15 nations, flies about 418 kilometres above Earth.

In addition to food, supplies and equipment, the Cygnus spacecraft was loaded with more than 725 kilograms of science experiments, including an investigation to chemically analyse meteors as they burn up in Earth's atmosphere.

The Cygnus also carried a prototype satellite owned by Washington-based startup Planetary Resources Inc, which is developing technology to mine asteroids.

The satellite, designated A3, was to be released into space by a commercially owned small spacecraft launcher aboard the station.